1. Field of the Invention
The invention is related to micro-electronics and more particularly to an on-chip stacked inductor having high quality factor for RF application.
2. Description of Related Art
At present, integrated circuits usually contain a lot of passive devices. One of the most important components in RF CMOS/BiCMOS integrated circuits is on-chip inductor. Inductors have great impact on the RF characteristic in common wireless products. The design and analysis for this component has been widely researched as a result. Nowadays, on-chip inductors with high Q factor are widely used in voltage controlled oscillator, low noise amplifier and other RF building blocks. On-chip stacked inductors can reduce the chip area in a large extent, thus reducing the production cost.
Quality factor (Q factor) of an inductor is a major factor to indicate the performance of the inductor. High Q factor leads to low magnetic loss and high efficiency of the inductor.
A conventional stacked inductor as shown in FIG. 1 is composed of a first coil in an upper metal layer and a second coil in a lower metal layer. Both coils are formed with only one layer of metal. Since the thickness of the lower metal layer is smaller than that of the upper metal layer, the resistance of the second coil is higher than the resistance of the first coil, which causes relatively high parasitic resistance. Although the above stacked structure can increase the inductance by more than two times compared with a single-layer inductor having the same chip area, the high resistance of the lower metal layer leads to the degradation of the Q factor. As a result, it could not meet the requirement of circuit design.